Anion Gap Calculator

Blood contains a mix of positively and negatively charged particles that must balance overall, but routine lab panels only measure some of them directly. The anion gap is a calculated stand-in for the unmeasured anions, mostly proteins, phosphates, and sulfates, and a gap outside the expected range is one of the first clues used to narrow down the cause of a metabolic acidosis¹.

What This Calculator Measures

This calculates the anion gap from your basic metabolic panel values, and optionally corrects it for albumin. It's a screening calculation used to help categorize the cause of a metabolic acidosis, not a diagnosis on its own.

The Formula

Anion gap = Sodium − (Chloride + Bicarbonate)

Some labs instead use Anion gap = Sodium + Potassium − (Chloride + Bicarbonate). Neither is universally "the" correct version. This calculator states clearly which formula was used rather than presenting one as the only correct number.

Albumin-corrected anion gap = Anion gap + 2.5 × (4.0 − measured albumin), since low albumin is itself an unmeasured anion, and low albumin can mask what would otherwise be an elevated gap².

How This Calculator Computes Your Result

Enter sodium, chloride, and bicarbonate, plus potassium if you want the potassium-inclusive version. If you enter albumin, we also show the albumin-corrected gap alongside the standard one.

Why the Reference Range Has Shifted Over Time

Reference ranges for the anion gap aren't fixed the way some lab values are. Older textbooks and teaching materials commonly cite a wider range, roughly 8 to 16 mEq/L, reflecting the older lab methods (like flame photometry) that were standard when the anion gap concept became widely used.

Many modern labs, using newer ion-selective electrode methods, now report a narrower range, often closer to 3 to 11 mEq/L. This is a genuine methodological shift, not just noise, which is why comparing your result to your own lab's reported reference range matters more than comparing it to a single fixed number quoted online.

A Worked Example

Say your panel shows sodium 138, chloride 104 and bicarbonate 22 mEq/L. The standard gap is 138 − (104 + 22) = 12 mEq/L. If you tick the potassium option and enter 4.0, the result becomes 138 + 4.0 − (104 + 22) = 16 mEq/L. That is why a potassium-inclusive result needs a potassium-inclusive reference range³.

Now add an albumin of 2.0 g/dL. The corrected gap is 12 + 2.5 × (4.0 − 2.0) = 17 mEq/L. A gap of 12 can look unremarkable, but once the low albumin is accounted for it may point to a high anion gap acidosis that would otherwise be missed³. The calculator applies the correction to any albumin you enter, so an albumin above 4.0 g/dL lowers the corrected gap slightly. Results are shown to one decimal place. Low albumin also affects total calcium, which the Corrected Calcium Calculator adjusts for.

What a High Anion Gap Can Point To

An elevated gap most often reflects a metabolic acidosis, but it can also come from a lab error, metabolic alkalosis, a high phosphate level or an abnormal blood protein (paraproteinemia)⁴. In a high anion gap metabolic acidosis, acids such as lactate, ketones or toxic metabolites build up and add unmeasured anions. The main causes clinicians consider include³:

• Lactic acidosis, for example from shock or sepsis, or linked to drugs such as metformin

• Ketoacidosis: diabetic, alcoholic or starvation

• Kidney failure, which retains sulfate, phosphate and other acids, and rhabdomyolysis

• Toxic alcohols such as methanol, ethylene glycol and propylene glycol

• Salicylate (aspirin) poisoning

• 5-oxoproline (pyroglutamic acid), linked to chronic acetaminophen use, and D-lactic acidosis in short bowel syndrome

A gap above 18 mEq/L almost always means an organic acid is present, while kidney failure on its own rarely pushes the gap that high³. When toxic alcohol poisoning is a concern, clinicians often check the osmolar gap as well³.

Normal and Low Anion Gap Results

A metabolic acidosis with a normal gap is also called hyperchloremic acidosis, because chloride rises as bicarbonate falls and the gap stays the same³. Diarrhea and renal tubular acidosis are the main causes, and large volumes of IV normal saline are another common one³.

A low gap most often points to a lab error or low albumin. Less often it reflects a paraproteinemia, as in multiple myeloma, or intoxication with lithium, bromide or iodide⁴.

Comparing the Gap With Bicarbonate

When the gap is high, clinicians often compare the rise in the gap with the fall in bicarbonate, called the delta ratio, to look for a second acid-base disorder at the same time³. Kraut and Madias note that this ratio varies between patients, so it cannot be relied on alone to detect a mixed disorder⁴. This calculator does not compute the delta ratio.